Oil and natural gas are often found together in the same reservoir. The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area.
Natural gas processing begins at a wellhead. Most natural gas production contains to varying degrees, small (two to eight carbons) hydrocarbon molecules in addition to methane. Although the molecules exist in a gaseous state at underground pressures, these molecules will become liquid and condense at normal atmospheric pressure. Collectively, they are called condensates or natural gas liquids (NGLs). The pipe-line quality natural gas received and transported by mainline transmission systems must meet quality standards specified by various pipeline companies. In general, the natural gas cannot contain, among other things, more than trace amounts of compounds such as water vapor, nitrogen, carbon dioxide, etc. In addition, the natural gas should be transported at a specified dew point temperature below which vaporized gas liquid in the mixture will tend to condense at pipeline pressure.
The processing of wellhead natural gas into pipeline-quality dry natural gas can involve several processes to remove and/or separate constituents such as oil, water, and compounds comprising sulfur and carbon dioxide, and NGLs (condensate). In many instances, pressure relief at the wellhead will cause a natural separation of gas from oil. For example, gravity can cause the separation of the gas hydrocarbons from the heavier oil in a conventional closed tank. In some cases, however, a multi-stage gas-oil separation process can be implemented to separate the gas stream from the crude oil. This equipment typically comprises a separator, a heater treater, storage tanks, circulating pumps and a facility for the storage or disposal of water that is produced with the oil.
At a typical production facility, the separated water and condensate constituents can be diverted from the heater treater unit to storage tanks. Specifically, the water can be collected in a water storage tank until it is removed and/or transported to a disposal facility. The condensate fraction can be collected in an oil tank until it is removed and/or transported to a wholesale oil buyer and/or refinery. Natural gas, having undergone separation from the water and condensate constituents, can be separated and routed through a pipeline, commonly known as a sales line and a meter, to a gas gathering system where it can be sent to a compressor station and/or a gas processing facility to be compressed, refined, and sold to gas marketing companies.
A heater treater unit, or condensate separator, requires energy in the form of heat to accomplish the separation of the gas, oil, and water fractions. In addition, the heaters ensure that the temperature of the natural gas does not drop too low and cause a hydrate to form with the water vapor content of the gas stream. This heat is typically provided for by burning natural gas, diverted from the treater unit's gas section, as “fuel gas”. Since natural gas was historically inexpensive, the oil and gas industry did not typically measure or economize the consumption of fuel gas in the treater unit. As the price of natural gas has increased, however, technologies for controlling unmeasured consumption can be more favored.
As stated above, gravity can cause the separation of the gas hydrocarbons from the heavier oil in a conventional closed tank. In addition, elevated temperatures effective for lowering oil viscosity and promoting phase separation can cause a significant percentage of these lighter hydrocarbons to volatilize while being stored in the condensate tank. These lighter hydrocarbons represent a gaseous volume in an oil tank. As gas pressure in the tank builds, these volatile hydrocarbons, including methane can vaporize and escape to the atmosphere. In some cases, it can become necessary to reduce the pressure by venting these gases through an outlet mounted at the top of the oil tank. Regulatory requirements for reducing raw gas emissions into the atmosphere have caused the industry to consider developing methods to reduce raw gas emissions from oil tanks. For example, gas combustors can be installed adjacent to production treater units to efficiently burn oil tank vent gas and avoid venting raw gas emissions to the atmosphere.
The disclosed device provides oil and gas producers an alternative to burning vent gas to avoid venting gas to atmosphere. The disclosed device also provides a method of economizing the consumption of fuel gas in the treater unit by recycling the oil tank vent gas to the heater treater to augment its primary fuel.